The present invention relates generally to an exposure apparatus which exposes a substrate, and more particularly to an exposure apparatus for fabricating devices such as semiconductor devices like ICs or LSIs, imaging devices like CCDs, display devices like liquid crystal panels, and detecting devices like magnetic heads, for example, by exposing a circuit pattern on a reticle onto a wafer.
A projection exposure apparatus has been conventionally used for an apparatus to fabricate devices using photolithography, where the exposure apparatus projects, exposes, and transfers a circuit pattern of a reticle (mask) as an original form to a wafer or glass plate as a substrate by using a projection optical system.
The projection optical apparatus exposes the wafer for example, by using a step-and-repeat method or step-and-scan method. The step-and-repeat type projection exposure apparatus is called a stepper and exposes one whole shot simultaneously onto the wafer, then moves the wafer stepwise every one shot so that the next shot comes to an area to be exposed. The step-and-scan type projection exposure apparatus called a scanner exposes the circuit pattern of the reticle onto the wafer by scanning the wafer continuously onto the reticle, and moves the wafer stepwise every one shot so that the next shot comes to the area to be exposed.
FIG. 13 is a schematic block diagram of a conventional step-and-scan type exposure apparatus 1000. The exposure apparatus 1000 has a plurality of illumination modes such as a small σ mode in small σ illumination condition, a large σ mode in large σ illumination condition, and an annular mode in an annular illumination condition.
The exposure apparatus 1000 calibrates, in advance, the output of the sensor 1500 which detects the illumination in each illumination mode of the wafer 1400 and the output of the integrated exposure sensor 1210 provided in the illumination optical system 1200, and stores the calibration results. More, the exposure apparatus detects the illumination distribution of the exposure area in each illumination mode by moving the sensor 1500 in two dimensions, adjusts and stores the opening shape of a variable stop 1280a so that the illumination distribution is a specific (generally, an even distribution) in scanning exposure (see Japanese Patent Publication applications No. 7-037774, and No. 2000-114164).
In a scanning exposure, a running field stop 1280b arranged near the variable stop 1280a regulates the illumination area of the reticle 1600 and the illumination area of the wafer 1400 optically conjugate with the reticle 1600, and is movable to an arrow P direction in synchronization with the reticle 1600 and the wafer 1400.
In the case that a specific illumination mode is designated and the exposure in the specific illumination mode is decided, a scanning speed V [mm/s] of a wafer stage 1450, an opening width W [mm] of the variable stop in scanning direction, and a laser emission frequency F [Hz] are decided to satisfy the equation “V=F*W/n”. Here, n is an exposure pulse count that is a laser pulse count emitted to a specific spot on the exposure surface in scanning exposure. The minimum value of the pulse count n is decided by the sensitivity of the reticle 1600, a pulse energy, and the like. Additionally, the exposure needs the minimum value of the pulse count or more.
The variable stop 1280a is arranged so as to moderately defocus (be distanced from) the running field stop 1280b (arranged at a position conjugate with the surface of the wafer 1400) to form the illumination distribution on the wafer 1400 in scanning desirably. Therefore, the integrated illumination (the illumination distribution of the shot area after scanning) is highly uniformed by scanning with forming the illumination distribution to be approximately trapezoid on the wafer 1400 surface in scanning direction while the stage stops (see Japanese Patent Publication application No. 60-158449).
It is proposed that the exposure apparatus moves the stop along the optical axis to be conjugate with the reticle when the thickness of the reticle has changed (see Japanese Patent Publication application No. 60-45252).
Along with recent demands for an exposure apparatus with higher resolution, shorter wavelengths of light sources have been promoted from i-line to KrF excimer lasers and ArF excimer lasers. The wavelengths of the light sources are thought to likely be shorter, such as F2 laser, in the future. Also, the numerical aperture (NA) of the exposure apparatus will be larger: from 0.70 to 0.80 or 0.85.
As previously explained, the variable stop is arranged so as to moderately defocus (be distanced from) the running field stop (arranged at a position conjugate with the surface of the wafer), so that the illumination distribution on the wafer surface is formed to be approximately trapezoid. Here, to form the trapezoid distribution (defocus level) appropriately, the distance between the variable stop 1280a and the running field stop 1280b should become shorter in accordance with the larger NA of the illumination light. However, the conventional exposure apparatus cannot bring the variable stop 1280a sufficiently close to the running field stop 1280b, because their distance has a mechanical limitation.
Therefore, if the NA of the illumination light is large, the trapezoidal illumination distribution on the wafer surface will be defocused more than necessary, the range of the trapezoid distribution will be forced out from the illumination area, the illumination efficiency will become low, and the productivity in the device fabrication will become low. More, as the scanning exposure uses the illumination light having the illumination distribution range that forces out from the illumination area, the uniformity of the integrated illumination (scanning illumination distribution) will deteriorate. Another problem is that defocus distortion is produced when the defocus level of the trapezoid distribution is different on one side of the scanning direction than on the other side of the scanning direction. Here, defocus distortion means an image shift in defocusing the image from the best focusing position of the projection optical system. The projection exposure apparatus needs a small defocus distortion. Because the section of the wafer comes to have a stepped shape as shown in FIG. 14 in accordance with laminating layers in a semiconductor fabrication process, the pattern-exposed position will shift in the upper stair and in the lower stair.
The slant of the light beam balanced in the center of the wafer surface of the projection exposure apparatus may cause the defocus distortion. As shown in FIG. 14, if the balance center CL of the light beam which forms an image at a spot P is inclined from a direction perpendicular to the wafer surface, the image-transferred position will shift when the defocus is produced. For example, a grid pitch of a grid pattern shown in FIG. 15A, which is projected on the step-shaped wafer shown in FIG. 14, is elongated at the stepped part of the wafer as shown in FIG. 15B. Thus, fidelity of the transferred image deteriorates and the circuit pattern cannot be transferred accurately. The exposure apparatus should have low defocus distortion as explained before, and should not generate the position shift of the image even if there is a step on the wafer surface caused by the lamination of the layers.
The defocus distortion corresponds to the slant of the centroid of the exposure light beam integrated while scanning the exposure in the scanning exposure apparatus.
Now a description will be given of the defocus distribution caused by the variable stop referred to in FIG. 13 and FIG. 16. The illumination optical system 1200 is designed, for example, so that the reticle 1600 is conjugate with the running field stop 1280b. Without using the variable stop 1280a, all lights from a secondary light source formed by an optical integrator 1250 are overlapped onto the running field stop 1280b by being condensed by a condensing lens 1260. Therefore, the illumination area on the running field stop 1280b is illuminated uniformly. The illumination area on the reticle that is arranged at the position optically conjugate with the running field stop 1280b is also illuminated uniformly.
With the entrance of variable stop 1280a, the light beam is eclipsed and the trapezoid shaped illumination distribution is formed as explained before. FIG. 16 clearly shows that a light beam is eclipsed by a light shielding element 1280a1 of the variable stop from the upper-left toward the lower-right among the incidental light into an area A (one oblique side of the trapezoid) of the variable stop's opening. Therefore, the centroid of the light amount at each spot in the area A directs from the lower-left toward the upper-right. Similarly, a light beam from the lower-left toward the upper-right among the incidental light into an area B (the other oblique side of the trapezoid) of the variable stop's opening is eclipsed by a light shielding element 1280a2 of the variable stop. Therefore, the centroid of the light amount at each spot in area B directs from the upper-left toward the lower-right.
The distance from the running field stop 1280b to the variable stop 1280a1 and that to the variable stop 1280a2 are different, and the width of area A and that of area B are different. In this case, the centroid of the integrated exposure light amount is tilted, and defocus distortion in the scanning exposure is generated.
The amount of defocus distortion can be calculated by using integral calculus if the exposure is continuous. L is the width of the variable stop's opening, Δz1 and Δz2 are the defocuses of the variable stop 1280a1 and 1280a2 respectively. From the running field stop 1280b, NA is the numerical aperture of the incidental light to the wafer on the wafer surface. β is a magnification from the running field stop to the wafer. Tan θ is the tilt of the centroid of the integrated exposure light in the scanning exposure and is described as NA2β2 (Δz1−Δz2)/(3L) in the approximation of tan θ=sin θ. For example, if NA is 0.8, β is 0.5, Δz1−Δz2 is 1 mm, and L is 7 mm, then tan θ as the tilt of the centroid of light will be 0.007619. Defocusing 1 μm shifts the image to 7.6 nm. That means if the line width of the transferred pattern is 70 nm, the non-allowable image shift of 10% of the line width is produced.